Vehicles, such as buses, are conventionally provided with their own, on board, electrical system for supplying electrical energy to an electric motor for cranking the engine in order to start it and for operating the lights and accessories of the vehicle. Large engines, particularly diesel engines, require that a very high starting torque be developed by the starting motor. Thus, they have a high power requirement.
Storage batteries for vehicles are conventionally manufactured in mass production having a 12 volt nominal battery voltage. Because of the savings of mass production techniques, such batteries are highly desirable for use on vehicles. Similarly, 12 volt lights and accessories are manufactured in high quantities and therefore are available most inexpensively.
Because, however, the electrical starting motors require high input power, as much as 10,000 watts during cranking, very high currents are developed during cranking which can produce substantial resistive heat losses thus wasting energy, lowering voltage in the circuit and reducing the voltage actually applied to the starting motor.
Designers have sought to reduce these resistance losses by designing starting motors which operate at 24 volts. These motors also are less massive than a 12 volt motor developing the same torque.
However, if the vehicle is operated with a simple 24 volt electrical system it requires lights and accessories designed for 24 volt operation. This creates a problem because 24 volt electrical lights and accessories are not as conveniently available as their 12 volt equivalents and are more expensive because there is such a smaller demand and therefore less cost reduction from mass production and marketing. Additionally, 24 volt lights, particularly headlamps, have a shorter lifetime than their 12 volt equivalents because their filaments are thinner and therefore more subject to damage from mechanical shock.
A commonly adopted solution has been to provide 24 volts for cranking the engine but 12 volts for all the lights and accessories. One proposed method for doing this is to provide two duplicate electrical systems, one operating at 24 volts and the other at 12 volts. Such systems are expensive to install and to maintain because they require a duplicate investment in batteries, generators, regulators and the like as well as twice as many devices subject to failure and requiring repair.
Another option is to utilize two 12 volt batteries connected in series. The series voltage is utilized for cranking and the 12 volt power system is just tapped off across one of these batteries. The difficulty with that system is that one battery which supplies the 12 volts becomes discharged while the other does not. The alternator or generator used to recharge the batteries is series connected to the two batteries to charge them both at the same time. Upon charging, the battery which was not discharged will become substantially overcharged, that is charged to a higher over voltage, and will start boiling and outgassing. This causes a short lifetime and frequent replacement.
One suggested solution to that problem is to attempt to split the light and accessory load into two equal loads, one applied to each battery. Thus, half the load is applied to one battery and half the load is applied to the other. The difficulty is that it is impossible to completely balance these loads to get an even distribution of current drain and therefore eventually one of the two batteries becomes relatively less charged than the other and the same destructive problems arise. With the split load technique, the destruction just takes longer.
Yet another proposed solution is to utilize a 24 volt source but connect a DC to DC converter to it to supply 12 volts to the lights and accessories. Although this system represents an improvement, nonetheless problems still remain. First, from ageing alone the electrical characteristics of batteries will vary so that two series batteries will eventually have different charge acceptances. Eventually one will become more charged relative to the other. Thus, even if neither of the two series batteries has a separate load, nonetheless one of the batteries will eventually become relatively overcharged and the same type of damage will occur as described above. Additionally, in a system which supplies electrical energy to lights and accessories through a converter, if the converter circuitry should fail, power will not be available for emergency use. This could be extremely dangerous if the converter would fail at night while a vehicle is travelling down the road with the driver depending upon the headlights for visibility. Additionally, some loads occasionally require a high surge current of short, brief time duration. In those systems utilizing a converter to supply power to the 12 volt lights and accessories, the converter must be designed to handle the surge current.
In addition to the problem of battery failures due to overcharging, in previous systems when one battery fails it has been necessary to replace both of them. The electrical characteristics of batteries change with time and usage and therefore the use of an old and a new battery together would soon result in quick destruction of the new battery because of their differing electrical characteristics.
There is therefore a need for a network for connection to a pair of series connected batteries so that the engine cranking power can be supplied by the total series voltage of the batteries, the light and accessory power can be tapped off a single battery and yet the battery voltages are equalized and the battery to the lights and accessories will be reliably available as a backup even in the event of a catastrophic failure of the circuitry.